Fuel tank and method for making same

ABSTRACT

In a fuel tank and a process for manufacturing the fuel tank, accessories emerging through the wall of the tank are covered by a flexible film of multilayer structure that includes a fuel barrier layer. The perimeter of the film is welded to the wall of the tank.

The present invention relates to a plastic fuel tank impermeable to gases and to liquids.

Plastic fuel tanks used in the industry, particularly in the automotive industry for equipping vehicles, generally include one or more accessories with which they are associated, such as aeration and venting valves, take-up siphons, various sensors and their various connections, gauge-and pumping modules, etc.

Moreover, fuel tanks must meet increasingly stringent sealing and permeability standards. The limits of permitted emissions have become so low that losses associated with leakages and with the permeability of the interfaces of the accessories with the tank have assumed a higher relative proportion in the total losses from the tank/accessories system.

It is known, from Patent Application WO 01/21428, to close off the openings in the multilayer plastic tank that are made for introducing or fitting an accessory in the tank by means of a sheet of multilayer structure that is compatible with that of the tank, this sheet being welded to the outer wall of the latter.

However, in that document it is disclosed that the sheets used have a stiffness close to that of the walls of the tank. It follows that these sheets cannot be easily used when the emergent portion of an accessory is too bulky, or when the emergent surface of the accessory is irregular.

In addition, despite the bulging that can be produced near the edge of the sheet in order to locally reduce the thickness of the plastic layers constituting the multilayer sheet, the level of fuel losses by emission at the point where the sheet is welded to the tank still often remains too high and sometimes even exceeds the very low limits imposed by the latest environmental standards.

It is an object of the invention to provide a plastic tank that limits even further the losses of fuel at the interfaces of the accessories with the environment and, in particular at the interfaces of the accessories with the tank, if these accessories pass through the wall of the tank.

Another advantage of the invention is that it allows a system for fixing the accessory to the tank which is independent of t he high impermeability requirements imposed on the tank/accessories assembly.

For this purpose, the invention relates to a plastic fuel tank provided with at least one accessory located at least partly on the outside of the tank, the accessory being covered by a protective device that improves the impermeability of the tank/accessories assembly, in which this device is a flexible film of multilayer structure that includes at least one fuel barrier layer, which film is fastened to the tank by welding it to the outer surface of the said tank over the entire periphery of the film.

The term “fuel tank” is understood to mean any type of tank capable of storing a liquid and/or gaseous fuel under varied temperature and pressure conditions. More particularly intended are tanks of the type of those encountered in motor vehicles. The term “motor vehicle” is understood to include cars, motorcycles and lorries.

The term “plastic” is understood to mean any material comprising at least one synthetic resin polymer.

All types of plastic may be suitable. Particularly suitable plastics belong to the group of thermoplastics.

The term “thermoplastic” is understood to mean any thermoplastic polymer, including thermoplastic elastomers, as well as blends thereof. The term “polymer” is understood to mean both homopolymers and copolymers (especially binary or ternary copolymers). Examples of such copolymers are, nonlimitingly, random copolymers, linear block copolymers, other block copolymers, and graft copolymers.

Any type of thermoplastic polymer or copolymer whose melting point is below the decomposition temperature is suitable. Synthetic thermoplastics having a melting range spread over at least 10 degrees Celsius are particularly suitable. Examples of such materials are those exhibiting polydispersion in their molecular mass.

In particular, polyolefins, vinyl polyhalides, thermoplastic polyesters, polyketones, polyamides and copolymers thereof may be used. A blend of polymers or copolymers may also be used, as may a blend of polymeric materials with inorganic, organic and/or natural fillers such as, for example, but not limitingly, carbon, salts and other inorganic derivatives, and natural or polymeric fibres. It is also possible to use multilayer structures consisting of stacked layers fastened together, comprising at least one of the polymers or copolymers described above.

Polyolefins have given good results. Among polyolefins, high-density polyethylene (HDPE) is preferred.

The invention applies to a tank provided with at least one accessory-located at least partly on the outside of the tank. The term “accessory” is understood to mean in general any member via which liquid or gas passes, or one in contact with liquid or gas, and which fulfils a particular function specific to the fuel device of which the tank forms part, including a function of transporting liquid and/or gas between two other members.

Examples of such accessories comprise, not limitingly, the following:

-   -   a container that contains any chemical or physical composition,         especially a vapour absorption canister;     -   a liquid or gas gauge;     -   an electrical connection terminating, in a liquid or gas gauge;     -   a liquid or gas pump;     -   a safety valve for controlled closure of the tank under certain         particular conditions;     -   a drainable container for receiving liquid;     -   an electrical connection for supplying the motor of a liquid or         gas pump;     -   a liquid pipe terminating in a supply device for any device,         especially an engine;     -   a liquid/vapour separation device.

Any combination of at least two accessories may be used, optionally in the presence of several examples of the same accessory.

According to the invention, the accessory is located at least partly on the outside of the tank. The accessory may be located completely on the outside of the tank. One example is a module mounted on a wall of the tank and fulfilling a particular function involving the presence of fuel. The accessory may also be located only partly outside the tank. In this case, it passes through the wall of the latter and is provided with an interface with this wall that ensures relative sealing with respect to gases and liquids.

According to the invention, the accessory is covered by a protective device which improves the impermeability of the tank/accessories assembly. In other words, the presence of the protective device significantly reduces the losses of fuel at the surface of the accessory and/or at the interface of the latter with the tank, if the accessory passes through the wall of the tank.

According to the invention, this device is a flexible film of multilayer structure, that is to say a laminated structure resulting from stacking several layers each comprising at least one thermoplastic, the thickness and the nature of the layers of which stack are such that the flexibility of the film is substantially greater than that of the wall of the tank. Preferably, the flexible film can be easily deformed, by simple manual action.

The flexible film has a surface of various types. For example, it may constitute a convex surface whose shape is defined by relatively circular or elliptical curved lines, or on the contrary has sharp angles such as rectangular or polygonal shapes. The flexible film may also have a non-convex surface that includes at least one convex surface located on the inside of the surface of the film. In each of these convex surfaces included in the non-convex surface of the film, the multilayer structure is interrupted in order to leave room for a void enclosed in the non-convex surface.

According to the invention, the flexible film includes at least one fuel barrier layer. The term “fuel barrier layer” is understood to mean a layer impermeable to gaseous and liquid fuels. The barrier layer generally includes a barrier resin. Any known barrier resin may be present in the barrier layer, provided that it is effective against fluid fuels liable to be in contact with the tank and/or the accessory, particularly hydrocarbons, and provided that it is compatible with the technique used to manufacture the multilayer flexible film structure.

Among possible resins, mention may be made, non-limitingly, of polyamides or copolyamides, and ethylene/vinyl alcohol random copolymers. A blend of various barrier resins is also possible. Very good results have been obtained with a barrier layer comprising a barrier resin made of an ethylene/vinyl alcohol random copolymer.

The flexible film may have been obtained by any known technique resulting in the manufacture of a thin flexible multilayer structure. One possible technique is the technique of extruding a multilayer film through a sheet die. Another possible technique is compression moulding a multilayer sheet.

The flexible film covering the accessory of the tank according to the invention is fastened to the tank by welding. The flexible film may, for example, form a pocket trapping an accessory, which is welded to the tank.

In the case of tanks according to the invention, the flexible film is welded locally to the outer surface of the tank, over the entire periphery of the film.

The welding of the flexible film may be carried out in a variety of ways. All types of welding compatible with the plastics to be assembled are suitable. Preferably, the welding is of the laser or infrared radiation welding type. In this case, the film may advantageously be welded over a broader peripheral region than with other types of welding. A width of 10 to 20 mm for example is possible with laser radiation welding.

In a preferred embodiment of the tank according to the invention, at least one layer of the flexible film that is not provided with a barrier function is thinned around the periphery of the film over a distance at least equal to the width of the welding region. In this embodiment, the thickness of the layer not provided with a barrier function is reduced around the periphery of the flexible film in such a way that, at least in the welded region on the wall of the tank, the cross section of the fuel permeation channel is reduced. Particularly preferably, the layer or layers not provided with a barrier function that are located between the tank and the barrier layer are thinned.

According to another embodiment compatible with the embodiments described above, the layer of the flexible film which is directly in contact with the wall of the tank comprises a composition opaque to the laser radiation or to the infrared radiation. Advantageously, this embodiment provides tanks in which the welding of the flexible film by means of laser radiation or infrared radiation is of higher quality. The expression “composition opaque to laser or infrared radiation” is understood to mean a composition that absorbs and/or reflects a substantial proportion of the laser or infrared radiation energy. An example of such a composition that has given good results is a carbon black filler. This composition is advantageously distributed uniformly throughout the polymeric material of the layer directly in contact with the wall of the tank.

Another embodiment of the tank according to the invention, compatible also with one or more of the embodiments described above, is that in which the flexible film comprises at least one layer containing a polyolefin. Preferably, the polyolefin is high-density polyethylene (HDPE). Particularly preferably, the polyethylene-containing layer also includes a composition opaque to laser radiation or infrared radiation. An HDPE layer containing carbon black has given excellent results.

The flexible film may completely cover the accessory so as to remove any direct contact between this accessory and the atmosphere outside the tank.

Alternatively, the flexible film may also be placed in such a way that the accessory comprises a pipe which opens to the atmosphere external to the tank and in such a way that the film allows only the pipe to run out into this atmosphere. In such a case, a means of fastening the film to the pipe at the outlet interface of the latter with the film is corporated. Various fastening means are possible, provided that they provide a sealing contact better than that at the interface of the end of the pipe located beneath the film with the tank or an accessory thereof. Fastening means that have given good results are a snap-fastening system and a system consisting of a threaded screw on the pipe and of a ring screwed onto the latter, trapping the film in the threads of the screw, between the ring and the thread on the pipe.

The invention also relates to a process for manufacturing a plastic fuel tank provided with at least one accessory connected to the internal space of the tank via at least one opening in the wall of this tank, the accessory and/or the opening being covered by an impermeable protective device, in which there is placed, over the accessory, a thin film of multilayer structure that icludes at least one fuel barrier layer, the film then being welded over its entire periphery to the tank by making laser or infrared radiation scan this peripheral region.

In this process, the common terms have the same meaning as that given above in the description of the tank.

According to one method of implementing the process according to the invention, the film is scored and preformed by thermoforming, independently of the fuel tank, before welding it to the tank by means of laser or infrared radiation.

The preforming of the flexible film by thermoforming consists in giving it a shape which, while still remaining flexible and consequently modifiable to a certain extent without substantial mechanical forces, approaches the shape of the outer surface of the accessory to be protected. Preferably, the preforming of the flexible film is carried out by plastic deformation.

According to another method of implementing the process according to the invention, compatible with the embodiments described above, before carrying out the welding, the film is pressed against the accessory and the wall of the tank in the vicinity of this accessory by means of a stream of hot air.

Advantageously, the stream of hot air may be delivered by means of a pipe with a diameter close to the width of the accessory, this pipe being directed towards this accessory.

The figures that follow have the purpose of illustrating the invention, without seeking to restrict its scope.

FIG. 1 illustrates a venting valve (1) of circular geometry, comprising a skirt (2) made of polyacetal. The valve (1) is held in place by being clipped onto a siphon (3), the nozzle of which is connected to a pipe for the internal ventilation of a multilayer fuel tank (4). A sheet (5) of multilayer structure comprising two HDPE layers trapping an EVOH barrier layer was welded by hot-plate welding to the external surface (6) of the wall of the tank (5). Embossments (7) and (8) were made in the sheet (5) and the surface (6) of the wall of the tank (4) so as to allow the barrier layers to come together during the welding of the sheet (5) to the tank (4) and the siphon (3). A three-layer multilayer film (9) of circular shape, with a total thickness of 0.3 mm, comprising an EVOH barrier layer sandwiched between two HDPE layers, the HDPE layer facing the tank (4) being filled with 0.25% by weight of carbon black, was positioned above the sheet (5) and welded by scanning with diode laser radiation used in pulsed mode (FAP-type YAG laser with a wavelength of 809 nm and a power of 35 W) over a circular region (10), (11) with a width of 16 mm of the outer wall of the tank surrounding the valve (1) and the siphon (3).

FIG. 2 illustrates the covering of a ring-nut-type stopper used for fitting a pump-gauge module for an HDPE fuel tank (103). A support plate (101) has been positioned over an opening (102) made in a tank (103). A ring-nut system (104) screwed onto an end-fitting (105) defining the walls of the opening (102) allows the mounting plate (101) to be fastened to the tank (103). A flexible seal (106) provides a first level of sealing.

A siphon (107) passing through the support plate (101) allows a fitting (108) to be fitted by snap-fastening onto the fuel outflow line.

A combination of the end-fitting (105), the ring-nut system (104) and the support plate (101) is covered by a 0.3 mm thick film (109) having a multilayer structure comprising an EVOH barrier layer sandwiched between two HDPE layers, the HDPE layer of which that is in contact with the tank is an HDPE layer filled with 0.3% by weight of carbon black. The film (109) is provided with an opening allowing passage of the siphon (107) and is welded by the technique of scanning by means of laser radiation of the same type and the same power as that used to weld the film described in FIG. 1. The welding was carried out at the external surface of the tank (103) over a circular region 15 mm in width. A seal (110) provides sealing around the siphon (107) during snap-fastening of the female part of the fitting (108). 

1-10. (Canceled).
 11. A plastic fuel tank comprising: at least one accessory located at least partly on an outside of the tank, the at least one accessory being covered by a protective device that improves impermeability of an assembly of the tank and the at least one accessory, wherein the protective device includes a flexible film of multilayer structure that includes at least one fuel barrier layer, which film is fastened to the tank by welding the film to an outer surface of the tank over an entire periphery of the film.
 12. The tank according to claim 11, wherein the welding is of laser or infrared radiation welding.
 13. The tank according to claim 11, wherein at least one layer that is not provided with a barrier function is thinned around the periphery of the film over a distance at least equal to a width of a welding region.
 14. The tank according to claim 11, wherein the layer of the film that is directly in contact with the wall of the tank comprises a composition opaque to a laser radiation or to infrared radiation.
 15. The tank according to claim 11, wherein the film comprises at least one layer containing a polyolefin.
 16. The tank according to claim 11, wherein the film completely covers the at least one accessory to remove any direct contact between the at least one accessory and atmosphere outside the tank.
 17. The tank according to claim 11, wherein the at least one accessory comprises a pipe that opens the atmosphere external to the tank, and wherein the film allows only the pipe to run out into the external atmosphere.
 18. A process for manufacturing a plastic fuel tank provided with at least one accessory connected to an internal space of the tank by at least one opening in the wall of the tank, comprising: covering the at least one accessory and/or the opening by an impermeable protective device; placing, over the at least one accessory, a thin film of multilayer structure that includes at least one fuel barrier layer; and welding the film over its entire periphery to the tank by making laser or infrared radiation scan a peripheral region.
 19. A process according to claim 18, wherein the film is scored and preformed by thermoforming, independently of the fuel tank, before welding the film to the tank by laser radiation or infrared radiation.
 20. The process according to claim 18, wherein, before the welding, the film is pressed against the at least one accessory and the wall of the tank in a vicinity of the at least one accessory by a stream of hot air. 